The present invention relates to a coordinate input device which employs a pressure-sensitive rubber sheet.
A coordinate input device using a resistance plate has recently been utilized to input graphic information, character information, etc. When pressure is applied onto the resistance plate in this conventional coordinate input device, a pressure-sensitive rubber sheet is conducted at the pressure applied point of the resistance plate. DC voltage is applied to the resistance plate in its X-axis and Y-axis directions alternately. When the pressure is applied to the pressure-sensitive rubber sheet by a probe, the coordinate information of the pressure applied point are taken as X-axis and Y-axis voltages through the pressure-sensitive rubber sheet, and the X-axis and Y-axis voltages are converted into digital information to be input to a computer.
In a conventional coordinate input device disclosed in Japanese patent early publication No. 53-149055 laid open on Dec. 26, 1978, a surface electrode plate, e.g., a thin metallic plate or an electrically conductive rubber sheet is placed on the upper surface of a pressure-sensitive rubber sheet, and the voltages at the pressure-applied point are taken from the surface electrode plate. This device can eliminate the necessity of a conductive probe to be connected with a lead wire and enables the coordinate information to be input to a computer with a pencil or a pen.
However, this device still has problems described below. In case where the surface electrode plate is formed of a metallic plate, since the metallic plate is weak for plastic deformation, the metallic plate can hardly restore when pressure is removed from the surface electrode plate. Therefore, the continuous input of coordinate information is difficult. Further, in case where the surface electrode plate is formed of an electrically conductive rubber sheet, it is difficult to obtain accurate coordinate information, since the resistance value of the conductive rubber is relatively large.